The investigations covered by this grant will primarily explore the capabilities of the high-voltage electron microscope for the study of biological systems. First, the instrument will be used in the stereomicroscopy of thick sections because three-dimensional information, normally acquired through elaborate and perturbing serial sectioning, is essential for morphological and functional problems in cell biology. Second, the increased penetration of high-energy electrons will be used in observations of hydrated specimens and especially of living cells or bacteria placed in an environmental stage. Third, the reduced damage of specimens and of the increased theoretical resolving power occurring at kinetic energies of about 1 meV will be explored in cases where these considerations are critical. Fourth, techniques of image recording will be extended from the photographic process to other mechanisms suggested by the characteristic interactions of 1 MeV-electrons with matter. Fifth and finally, image carrier and digitized image intensifying systems will be evaluated for the observation of low intensity pictures and of transient motions in the environmental cell. It is therefore justified to anticipate a substantial wealth of information in all these directions. The installation will be made available to investigators from other institutions and also from other disciplines.